The present invention relates to purine nucleoside analogues containing a carbocyclic ring in place of the sugar residue, pharmaceutically acceptable derivatives thereof, and their use in medical therapy, particularly for the treatment of certain viral infections.
Hepatitis B virus (HBV) is a small DNA containing virus which infects humans. It is a member of the class of closely related viruses known as the hepadnaviruses, each member of which selectively infects either mammalian or avian hosts, such as the woodchuck and the duck.
Worldwide, HBV is a viral pathogen of major consequence. It is most common in Asian countries, and prevalent in sub-Saharan Africa. The virus is etiologically associated with primary hepatocellular carcinoma and is thought to cause 80% of the world""s liver cancer. In the United States more than ten thousand people are hospitalized for HBV illness each year, an average of 250 die with fulminant disease.
The United States currently contains an estimated pool of 500,000-1 million infectious carriers. Chronic active hepatitis will develop in over 25% of carriers, and often progresses to cirrhosis. It is estimated that 5000 people die from HBV-related cirrhosis each year in the USA, and that perhaps 1000 die from HBV-related liver cancer. Even when a universal HBV vaccine is in place, the need for effective anti-HBV compounds will continue. The large reservoir of persistently infected carriers, estimated at 220 million worldwide, will receive no benefit from vaccination and will continue at high risk for HBV-induced liver disease. This carrier population serves as the source of infection of susceptible individuals perpetuating the instance of disease particularly in endemic areas or high risk groups such as IV drug abusers and homosexuals. Thus, there is a great need for effective antiviral agents, both to control the chronic infection and reduce progression to hepatocellular carcinoma.
Clinical effects of infection with HBV range from headache, fever, malaise, nausea, vomiting, anorexia and abdominal pains. Replication of the virus is usually controlled by the immune response, with a course of recovery lasting weeks or months in humans, but infection may be more severe leading to persistent chronic liver disease as outlined above. In xe2x80x9cViral Infections of Humansxe2x80x9d (second edition, Ed., Evans, A. S. (1982) Plenum Publishing Corporation, New York), Chapter 12 describes in detail the etiology of viral hepatitis infections.
Of the DNA viruses, the herpes group is the source of many common viral illnesses in man. The group includes cytomegalovirus (CMV), Epstein-Barr virus (EBV), varicella zoster virus (VZV), herpes simplex virus (HSV) and human herpes virus 6 (HHV6).
In common with other herpes viruses, infection with CMV leads to a life-long association of virus and host and, following a primary infection, virus may be shed for a number of years. Clinical effects range from death and gross disease (microcephaly, hepatosplenemegaly, jaundice, mental retardation) through failure to thrive, susceptibility to chest and ear infections to a lack of any obvious ill effect. CMV infection in AIDS patients is a predominant cause of morbidity as, in 40 to 80% of the adult population, it is present in a latent form and can be reactivated in immunocompromised patients.
EBV causes infectious mononucleosis and isalso suggested as the causative agent of nasopharyngeal cancer, immunoblastic lymphoma, Burkitt""s lymphoma and hairy leukoplakia.
VZV causes chicken pox and shingles. Chicken pox is the primary disease produced in a host without immunity. In young children, it is usually a mild illness characterized by a vesicular rash and fever. Shingles is the recurrent form of the disease which occurs in adults who were previously infected with varicella. The clinical manifestations of shingles include neuralgia and a vesicular skin rash that is unilateral and dermatomal in distribution. Spread of inflammation may lead to paralysis or convulsionsand coma can occur if the meninges becomes affected. In immunodeficient patients, VZV may disseminate causing serious or even fatal illness.
HSV 1 and HSV 2 are some of the most common infectious agents of man. Most of these viruses are able to persist in the host""s neural cells. Once infected, individuals are at risk of recurrent clinical manifestation of infection which can be both physically and psychologically distressing. HSV infection is often characterized by extensive lesions of the skin, mouth and/or genitals. Primary infections may be subclinical although they tend to be more severe than infections in individuals previously exposed to the virus. Ocular infections by HSV can lead to keratitis or cataracts. Infection in the newborn, in immunocompromised patients or penetration of infection into the central nervous system can prove fatal. HHV6 is the causative agent of roseola infantum (exanthum subitum) in children which is characterized by fever and the appearance of a rash after the fever has dedined. HHV6 has also been implicated in syndromes of fever and/or rash and pneumonia or hapatitis in immunocompromised patients.
It has been reported thatthe carbocyclic anaiogue of 2xe2x80x2-deoxyguanosine (2xe2x80x2-CDG) i.e. (1R*, 3S*, 4R)-2-amino-1,9-dihydro-9-[3-hydroxy-4-hydroxymethyl)cyclopentyl]-6H-purine-6-one, is active against several viruses. Thus in Proc. Natl. Acad. Sci. USA 1989, Vol. 86, pp 8541-8544, it is disclosed that 2xe2x80x2-CDG inhibits hepatitis B viral replication. 1. Med. Chem (1987) 30, pp 746-749 and Biochemical Pharmacology (1990) Vol. 40, No. 7, pp 1515-1522, report 2xe2x80x2-CDG, especially the (+)-enantiomer, as active against herpes simplex virus type 1 (HSV-1). Furthermore 2xe2x80x2-CDG and general analogues thereof are disclosed together with a plurality of other compounds in the following patent publications: U.S. Pat. No. 4,543,255 (with reference to HSV 1 and 2), PCT 90/06671 (with reference to hepatitis B), EP 219838, PCT 91/13549 (with reference to cytomegalovirus (CMV)). Other publication relating to 2xe2x80x2-CDG and the preparation thereof are J. Med. Chem. (1984) 27, pp 1416-1421, and J. Chem. Soc. Chem. Commun. (1987) pp 1083-1084.
It has now been discovered that certain analogues of 2xe2x80x2-CDG as referred to below, are useful for the treatment or prophylaxis of certain viral infections. According to a first aspect of the present invention, novel compounds of the formula (I) are provided: 
wherein R1 represents
hydrogen;
C3-8 alkenyloxy; C3-8 cycloalkoxy (e.g. cyclopentoxy); C4-8 cycloalkenyloxy (e.g. cyclopenten-3-yloxy); aryloxy (e.g. phenoxy) or arylalkoxy (e.g. benzyloxy) in which the aryl may be substituted with one or more C1-4 alkyl, halogen, hydroxy, C1-4 alkoxy, amino or nitro;
C3-6 alkenylthio (e.g. allylthio); C3-6 cycloalkenylthio; C4-8 cycloalkenylthio; arylthio (e.g. phenyithio) or arylalkylthio (e.g. benzylthio) in which the aryl may be substituted with one or more C1-4 alkyl, halogen, hydroxy, C1-4 alkoxy, amino or nitro;
an amino group, xe2x80x94NR2R3, in wnich R2 and R3 may be the same or different and are independently selected from hydrogen; C1-8 alkyl; C1-6 alkoxy; C1-6 hydroxyalkyl (e.g. hydroxyethyl); C1-6 alkoxyalkyl (e.g. methoxyethyl); C3-7 cycloalkyl (e.g. cyclopropyl, cyclobutyl or cyctopentyl) in which the cycloalkyl may be substituted by one or more C1-6 alkyl or hydroxy; aryl (e.g. phenyl) or aralkyl (e.g. benzyl) in which the aryl may be substituted with one or more C1-4 alkyl, halogen, hydroxy, C1-4 alkoxy, amino or nitro; C3-6 alkenyl (e.g. allyl); or R2 and R3 together form a 4- to 8-membered ring (e.g. azetidinyl or pyrrolidinyl); provided that R2 and R3 cannot both be hydrogen or both be C1-8 alkyl;
4-morpholinyl, 1-piperazinyl or 1-pyrrallyl;
or a pharmaceutically acceptable derivative thereof.
It is to be understood that the present invention encompasses the particular enantiomers depicted in formula (I), including tautomers of the purine, alone and in combination with their mirror-image enantiomers which are not depicted. Enantiomers depicted by formula (I), the xe2x80x9crelevantxe2x80x9d enantiomers, are preferred and more preferably the relevant enantiomer is provided substantially free of the corresponding enantiomer to the extent that it is generally in admixture with less than 10% w/w, preferably less than 5% w/w, more preferably less than 2% w/w and most preferably less than 1% w/w of the corresponding enantiomer based on the total weight of the mixture.
However, the processes disclosed pertain to the preparation opposite enantiomers via Examples 11-17 and Example 34.
Where reference herein is madeto an alkyl moiety, this includes methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, tert-butyl, n-pentyl, iso-pentyl, neopentyl and hexyl.
Furthermore reference to C3-7 cycloalkyl includes cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and cycloheptyl.
Preferably R1 represents a C3-7 cycloalkylamino most preferably cyclopropyl.
Particularly preferred examples of compounds of formula (I), exhibiting decreased toxicity compared to 2xe2x80x2-CDG, are:
a) (+)-(1S, 2R, 4R)-4-(2-amino-6-(cyclopropylamino)-9H-purin-9-yl)-2-(hydroxymethyl)-1-cyclopentanol
b) (+)-(1S, 2R, 4S)-4-(2-amino-6-(cyclopropyimethylamino)-9H-purin-9yl)-2-(hydroxymethyl)-1-cyclopentanol
c) (+)-(1S, 2R, 4R)-4-(2-amino-6-(1-pyrrolidinyl)-9H-purin-9-yl)-2-(hydroxymethyl)-1-cyclopentanol
d) (+)-(1S, 2R, 4R)-4-[6-(allylthio)-2-amino-9H-purin-9-yl]-2-(hydroxymethylg)-1-cyclopentanol
e) (+)-(1S, 2R, 4R)-4-(2-amino-6-(cyclopentyloxy)-9H-purin-9-yl)-2-(hydroxymethyl)-1-cyclopentanol
f) (+)-(1S, 2R, 4R)-4-(2-amino-(1-azietidinyl)-9H-purin-9yl)-2-(hydroxymethyi)-1-cyclopentanol
and pharmaceutically acceptable salts thereof.
The compounds of formula (I) above and their pharmaceutically acceptable derivatives are herein referred to asthe compounds according to the invention.
In a further aspect of the invention there are provided the compounds according to the invention for use in medical therapy particularly for the treatment or prophylaxis of viral infections such as hepadnaviral infections and herpes viral infections. To date compounds of the invention has been shown to be active against hepatitis B virus (HBV) and cytomegalovirus (CMV) infections, although early results also suggest that the invention could also be active against other herpes virus infections such as EBV, VZV, HSVI and II and HHHV6.
Other viral conditions which may be treated in accordance with the invention have been discussed in the introduction hereinbefore.
In yet a further aspect of the present invention there is provided:
a) A method for the treatment or prophylaxis of a hepadnaviral infection such as hepatitis B or a herpes viral infection such as CMV which comprises treating the subject with a therapeutically effective amount of a compound according to the invention.
b) Use of a compound according to the invention in the manufacture of a medicament for the treatment or propnylaxis of any of the above-mentioned infections or conditions.
By xe2x80x9ca pharmaceutically acceptable derivativexe2x80x9d is meant any pharmaceutically or pharmacologically acceptable salt, ester or salt of such ester of a compound according to the invention, or any compound which, upon administration to the recipient, is capable of providing (directly or indirectly) a compound according to the invention, or an antivirally active metabolite or residue thereof.
Preferred esters of the compounds of the invention include carboxyiic acid esters in which the non-carbonyl moiety of the ester grouping is selected from straight or branched chain alkyl, e.g. n-propyl, t-butyl, n-butyl, alkoxyalkyl (e.g. methoxymethyl), aralkyl (e.g. benzyl), aryloxyalkyl (e.g. phenoxymethyl), aryl (e.g. phenyl optionally substituted by halogen, C1-4 alkyl or C1-4 alkoxy or amino); sulfonate esters such as alkyl- or aralkylsulfonyl (e.g. methanesulfonyl); amino acid esters (e.g. L-valyl or L-isoleucyl); and mono-, di- or triphosphate esters. The phosphate esters may be further esterified by, for example, a C1-20 alcohol or reactive derivativethereof, or by a 2,3-di(C2-24)acyl glycerol.
With regard to the above-descrbed esters, unless otherwise specified, any alkyl moiety presentadvantageously contains 1 to 18 carbon atoms, particularly 3 to 6 carbon atoms such as the pentanoate. Any aryl moiety present in such esters advantageously comprises a phenyl group.
Any reference to any of the above compounds also includes a reference to a pharmaceutically acceptable salt thereof.
Pharmaceutically acceptable salts include salts of organic carboxylic acids such as acetic, lactic, tartaric, malic, isethionic, lactobionic, p-aminobenzoic and succinic adds; organic sulfonic acids such as methanesulfonic, ethanesulfonic, benzenesulfonic and p-toluenesulfonic acids and inorganic adds such as hydrochloric, sulfuric, phosphoric and sulfamic acids.
The above compounds according to the invention and their pharmaceutically acceptable derivatives may be employed in combination with other therapeutic agents for the treatment of the above infections or conditions. Examples of such further therapeutic agents include agents that are effective for the treatment of viral infections or associated conditions such as acyclic nucleosides (e.g. acyclovir), immunomodulatory agents such as thymosin, ribonucleotide reductase innibitors such as 2-acetylpyridine 5-[(2-chloroanilino)thiocarbonyl)]thiocarbonohydrazone, interferons such as xcex1-interferon. 1-xcex2-D-arabinofuranosyl-5-(1-propynyl)uracil, 3xe2x80x2-azido-3xe2x80x2-deoxythymidine, ribavirin and phosphonotormic acid. The component compounds of such combination therapy may be administered simultaneously, in either separate or combined formulations, or at different times, e.g. sequentially such that a combined effect is achieved.
The compounds according to the invention, also referred to herein as the active ingredient, may be administered for therapy by any suitable route including oral, rectal, nasal, topical (including transdermal, buccal and sublingual), vaginal and parenteral (including subcutaneous, intramuscular, intravenous and intradermal). It will be appreciated that the preferred route will vary with the condition and age of the recipient, the nature of the infection and the chosen active ingredient.
In general a suitable dose for each of the abovementioned conditions will be in the range of 0.01 to 250 mg per kilogram body weight of the recipient (e.g. a human) per day, preferably in the range of 0.1 to 100 mg per kilogram body weight per day and most preferably in the range 1.0 to 20 mg per kilogram body weight per day. (Unless otherwise indicated, all weights of active ingredient are calculated as the parent compound of formula (I); for salts or esters thereof, the weights would be increased proportionally.) The desired dose is preferably presented as two, three, four, five, six or more sub-doses administered at appropriate intervals throughout the day. These sub-doses may be administered in unit dosage forms, for example, containing 10 to 1000 mg, preferably 20 to 500 mg, and most preferably 100 to 400 mg of active ingredient per unit dosage form.
Ideally, the active ingredient should be administered to achieve peak plasma concentrations of the active compound of from about 0.025 to about 100 xcexcM, preferably about 0.1 to 70 xcexcM, most preferably about 0.25 to 50 xcexcM. This may be achieved, for example, by the intravenous injection of a 0.1 to 5% solution of the active ingredient, optionally in saline, or orally administered as a bolus containing about 0.1 to about 250 mg/kg of the active ingredient. Desirable blood levels may be maintained by a continuous infusion to provide about 0.01 to about 5.0 mg/kg/hour or by intermittent infusions containing about 0.4 to about 15 mg/kg of the active ingredient.
While it is possible for the active ingredient to be administered alone it is preferable to present it as a pharmaceutical formulation. The formulations of the present invention comprise at least one active ingredient, as defined above, together with one or more accentable carriers thereof and optionally other therapeutic agents. Each carrier must be xe2x80x9cacceptablexe2x80x9d in the sense of being compatible with the other ingredients of the formulation and not injurious to the patient. Formulations include those suitable for oral, rectal, nasal, topical (including transdermal buccal and sublingual), vaginal or parenteral (including subcutaneous, intramuscular, intravenous and intradermal) administration. The formulations may conveniently be presented in unit dosage form and may be prepared by any methods well known in the art of pharmacy. Such methods include thestep of bringing into association the active ingredient with the carrier which constitutes one or more accessory ingredients. In general, the formulations are prepared by uniformly and intimately bringing into association the active ingredient with liquid carriers or finely divided solid carriers or both, and then if necessary shaping the product.
Compositions suitable for transdermal administration may be presented as discrete patches adapted to remain in intimate contact with the epidermis of the recipient for a prolonged period of time. Such patches suitably contain the active compound 1) in an optionally buffered, aqueous solution or 2) dissolved and/or dispersed in an adhesive or 3) dispersed in a polymer. A suitable concentration of the active compound is about 1% to 25%, preferably about 3% to 15%. As one particular possibility, the active compound may be delivered from the patch by electrotransport or iontophoresis as generally described in Pharmaceutical Research, 3 (6), 318 (1986).
Formulations of the present invention suitable for oral administration may be presented as discrete units such as capsules, cachets or tablets each containing a predetermined amount of the active ingredient; as a powder or granules, as a solution or a suspension in an aqueous or non-aqueous liquid; or as an oil-in-water liquid emulsion or a water-in-oil liquid emulsion. The active ingredient may also be presented as a bolus, electuary or paste.
A tablet may be made by compression or molding, optionally with one or more accessory ingredients. Compressed tablets may be prepared by compressing in a suitable machine the active ingredient in a free-flowing form such as a powder or granules, optionally mixed with a binder (e.g. povidone, gelatin, hydroxypropyimethyl cellulose), lubricant, inert diluent, preservative, disintegrant (e.g. sodium starch glycollate, cross-linked povidone, cross-linked sodium carboxymethyl cellulose) surface-active or dispersing agent. Molded tablets may be made by molding in a suitable machine a mixture of the powdered compound moistened with an inert liquid diluent. The tablets may optionally be coated or scored and may be formulated so as to provide slow or controlled release of the active ingredient therein using, for example, hydroxypropyimethyl cellulose in varying proportions to provide the desired release profile. Tablets may optionally be provided with an enteric coating, to provide release in parts of the gut other than the stomach.
Formulations suitable for topical administration in the mouth include lozenges comprising the active ingredient in a flavored basis, usually sucrose and acacia ortragacanth; pastilles comprising the active ingredient in an inert basis such as gelatin and glycerin, or sucrose and acacia; and mouthwashes comprising the active ingredient in a suitable liquid carrier.
Formulations for rectal administration may be presented as a suppository with a suitable base comprising for example cocoa butter or a salicylate.
Formulations suitable for vaginal administration may be presented as pessaries, tampons, creams, gels, pastes, foams or spray formulations containing in addition to the active ingredient such carriers as are known in the art to be appropriate.
Formulations suitable for parenteral administration include aqueous and non-aqueous isotonic sterile injection solutions which may contain anti-oxidants, buffers, bacteriostats and solutes which render the formulation isotonic with the blood of the intended recipient; and aqueous and non-aqueous sterile suspensions which may include suspending agents and thickening agents. The formulations may be presented in unit-dose or multidose sealed containers, for example, ampules and vials, and may be stored in a freeze-dried (lyophilized) condition requiring only the addition of the sterile liquid carrier, for example water for injections, immediately prior to use. Extemporaneous injection solutions and suspensions may be prepared from sterile powders, granules and tablets of the kind previously described.
Preferred unit dosage formulations are those containing a daily dose or unit, daily sub-dose, as herein above recited, or an appropriate fraction thereof, of an active ingredient.
It should be understood that in addition to the ingredients particularly mentioned above the formulations of this invention may include other agents conventional in the art having regard to the type of formulation in question, for example, those suitable for oral administration may include such further agents as sweeteners, thickeners and flavoring agents
The present invention further incudes the following process for the preparation of compounds of formuia (I) above and derivatives thereof either alone or in combination with their corresponding enantiomers. The process according to the present invention comprises treating a compound of formula (Ia) either alone or in combination with its enantiomer (wherein Z represents a precursor group forthe said R1 group, R1 defined as in formula (I)) 
The conversion of (Ia) to (I) may be carried out in a conventional manner, for example, by treatment of a compound of formula (Ia) in which Z represents a leaving group (e.g. a halo such as a chloro group) with an appropriate amine (e.g. methylamine or dimethylamine) or an appropriate alkoxide (e.g. sodium methoxide or potassium n-butoxide) or an appropriate alkylsulfide (e.g. sodium methylmercaptide) or with sodium hydrogen sulfide orthicurea to provide the 6-thiopurine (R1xe2x95x90mercapto) which is then alkylated with appropriate alkylating agents (e.g. n-propyl iodide, allyl chloride, and dimethyl sulfate) in the presence of an equivalent of base (e.g. sodium hydroxide or potassium t-butoxide) to provide the corresponding alkylthio compoundsof formula (I).
The compounds of formula (Ia) employed as starting materials in the above process may be prepared by reacting a compound of formula (II) either alone or in combination with its enantiomer (wherein Z is defined as in formula (Ia) and R4 and R5 are either the same or differentand may be either hydrogen, formyl, or an amino protecting group such as a C2-6 alkanoyl, e.g. acetyl or isobutyryl, or C1-6 alkoxycarbonyl, e.g. tert-butoxycarbonyl) with a reactive derivative of formic acid (e.g. triethylorthoformate or diethoxymethyl acetate) optionally with a cosolvent such as dimethyiacetamide or dimethylformamide. 
It is understood that when R5 is other than hydrogen or formyl, deprotection preferably by prior treatment with dilute aqueous mineral acid is required priorto treatment with a reactive derivative of formic acid. In these cases the resulting mineral acid salt of (II) with R4 and R5 being hydrogen is efficiently converted directly to compounds of formula (Ia) by treatment with derivatives of formic acid. e.g. triethylorthoformate, preferably at 25xc2x0 C. for several hours. When other compounds of formula (II) are reacted with derivatives of formic acid, the reaction is conveniently effected by the addition of slightly more than one equivalent of a strong anhydrous acid, e.g. with 1.1 equivalents of ethanesulfonic acid per equivalent of (II) or 4 equivalents of concentrated aqueous hydrochloric acid per equivalent of (II), preferably at 25xc2x0 C. It is understood that subsequent treatment of the resulting products with dilute aqueous acid, e.g. 1N hydrochloric acid at 25xc2x0 C. for several hours cleaves derivatives formed, for example, by reaction of the hydroxy groups with triethyorthoformate.
The compounds of formula (II) employed as starting materials in the above process may be prepared by reacting a compound of formula (IIIa) either alone or in combination with its enantiomer with an appropriately substituted pyrimidine, e.g. 2,5-diamino-4,6-dichloropyrimidine or preferably derivatives thereof, e.g., N-(4,6-dichlor-5-formamido-2-pyrimidinyl) isobutyramide as described in EP 434450, Jun. 26, 1991. This reaction is preferably carried out at 80-120xc2x0 C., e.g. at reflux in n-butanol or t-butanol with 1-2 equivalents of a base, e.g. triethylamine or potassium carbonate. for 1 to 3 hours. 
The compounds of formula (IIIa), either alone or in combination with their enantiomers, employed as starting materials as described above may be prepared for example by deprotection of protected compounds of formula (IIIb) by methods known in the art (T. W. Greene, xe2x80x9cProtective Groups in Organic Synthesis,xe2x80x9d Wiley, New York, 1981, pp 218-287; J. F. W. McOmie, xe2x80x9cProtective Groups in Organic Chemistry,xe2x80x9d Plenum Press, New York, 1973, pp 43-93)
Most preferably, when R6xe2x95x90tert-butoxycarbonyi (BOC), deprotection can be achieved by the reaction with an acid of pKa less than three as is known in the art and is exemplified below. The amino diol of structure (IIIa) is so obtained in the form of its salt, which is suitable for use in the reaction to prepare compounds of formula (I). The free base of the amino diol (IIIa) is obtained, for example, by contacting the salt with a quaternary ammonium-type anion exchange resin in its hydroxide form as is exemplified below.
The compounds of formula (IIIb) empioyed as starting materials as described above may be prepared for example by desilylation of protected compounds of formula (IVa) by reaction with fluoride ion, as is known in the art and exemplified below (T. W. Greene, xe2x80x9cProtective Groups in Organic Synthesis,xe2x80x9d Wiley, New York, 1981, pp 218-287; J. F. W. McOmie, xe2x80x9cProtective Groups in Organic Chemistry,xe2x80x9d Plenum Press, New York, 1973, pp. 43-93; W. T. Markiewicz, Tetrahedron Letters 1980, 21 4523-4524); W. T. Markiewicz and M. Wiewiorowski, Nucleic Acids Research Special Publication No. 4, 3185-3188; W. T. Markiewicz, Chem. Research (S) 1979, 24-25; C. H. M. Verdegaal, P. L. Jansse, J. F. M. deRooij, G. Veeneman and J. H. vanBoom, Recueil 1981, 100, 200-204; C. Gioeli, M. Kwiatkowski, B. Oberg and J. B. Chattopadhyaya, Tetrahedron Letters 1981, 22, 1741-1744). 
It is surprising in view of the prior art that substantially less than two equivalents of fluoride ion is sufficient to effect deprotecton. in the presentcase approximately one equivalent of tetraethylammonium fluoride was found to be sufficient.
The compounds of formula (IVa) employed as starting materials as described above may be prepared for example by first thiocarbonation of the compounds of formula (IVc) to prepare thiocarbonates of formula (IVb), then reduction of the thiocarbonates of formula (IVb) with e.g. tributyltin hydride as is known in the art (M. J. Robins and J. S. Wilson, J. Am. Chem. Soc. 1981, 103, 932-933; M. J. Robins, J. S. Wilson and F. Hansske, J. Am. Chem. Soc. 1985, 105, 4059-4065; W. Hartwig, Tetrahedron 1993, 39, 2609-2645 and references therein, D. H. R. Barton, D. Crich, A. Lobberding and S. Z. Zard, Tetrahedron 1986, 42, 2329-2338; D. H. R. Barton and S. W. McCombie, J. Chem. Soc., Perkin I 1975, 1574-1585; D. H. R. Barton, W. B. Motherwell and A. Stange, Synthesis 1981, 743-745; N. Katagiri, M. Nomura, M. Muto and C. Kaneko, Chem. Pharm. Bull. 1991, 39, 1682-1688) and is exemplified below.
The compounds of formula (IVc) employed as starting materials as descri bed above may be prepared for example by selective protection through reaction of compounds of formula (V) with 1,3-dichloro-1,1,3,3-tetraisopropyl disiloxane as is known in the art (W. T. Markiewicz, N. S. Padyukova, Z. Samek, J. Smrt, Collection Czechosiov. Chem. Commun. 1980, 45, 1860-1865; W. T. Markiewicz, Tetrahedron Letters 1980, 21, 4523-4524; W. T. Markiewicz and M. Wiewiorowski, Nucleic Acids Research Special Publication No. 4, 3185-3188; W. T. Markiewicz, J. Chem. Research (S) 1979, 24-25; C. H. M. Verdegaal, P. L. Jansse, J. F. M. deRooij, G. Veeneman and J. H. vanBoom, Recueil 1981, 100, 200-204; C. Gioeli, M. Kwiatkowski, B. Oberg and J. B. Chattopadhyaya, Tetrahedron Letters 1981, 22, 1741-1744) and is exemplified below. 
The compounds of formula (V) employed as starting materials asdescribed above may be prepared for example by cis-dihydroxylation of compounds of structure (VIIa) using a catalytic amount of osmium tetroxide and N-methyl morphotine-N-oxide as is known in the art (V. VanRheenen, R. C Kelly and D. Y. Cha, Tetrahedron Letters 1976, 1973-1976; M. Schroder, Chem. Rev. 1980, 80, 187-213). The cis-hydroxylation reaction results in a mixture of two geometrical isomers of structure (VI) and (V). The separation of these isomers can be achieved by conventional methods such as chromatography or selective crystallization. The isomers wherein R6 is BOC are most easily separated by crystallization, as is exemplified below. 
The compounds of formula (VIlIa) employed as starting materials as described above maybe prepared for example by protection of the compound of formula (VIIb) by methods known in the art (T. W. Greene, xe2x80x9cProtective Groups in Organic Synthesis,xe2x80x9d Wiley, New York, 1981, pp. 218-287; J. F. W. McOmie, xe2x80x9cProtective Groups in Organic Chemistry,xe2x80x9d Plenam Press, New York, 1973, pp 43-93). Preferred are R6xe2x95x90C2-6 alkanoyl (e.g. acetyl) and C2-6 alkyloxycarbonyl (e.g. tert-butoxy carbonyl, BOC). Most preferred is R6xe2x95x90BOC, which is exemplified below.
The resolved (xe2x88x92)-amino alcohol of formula (VIIb) or a protected derivative (VIIa) can now be used to synthesize resolved carbocyclic nucleosides, (e.g. (1S, 4R)-4-(2-amino-6-(cyclopropylamino)-9H-purin-9-yl)-2-cyclopentene-1-methanol) as illustrated in EP 434450 (U.S. Pat. No. 5,087,697) and in the examples hereinafter. Thus, an enantiomer of carbocyclic nucleoside is obtainable by applying reactions that form the corresponding pyrimidine or purine base of the desired nucdeoside, as in known in the art and illustrated herein.
It will be appreciated that the stepsfrom formation of the resolved (xe2x88x92)-amino alcohol of formula (VIIb) up to formation of (1S, 4R)-4-(2-amino-6-(cyclopropylamino)-9H-purin-9-yl)-2-cyclopentene-1-methanol as described in EP 434450 (U.S. Pat. No. 087,697) are incorporated herein by reference, in particular Examples 1-5, 15-19, 26-28 and described herein (Examples 30-33).
Another aspect of the present invention includes a process for the preparation of (xe2x88x92)-(1S, 4R)-4-amino-2-cyclopentene-1-methanol, compound (VIIb), its mirror image enantiomer and mixtures of such enantiomers. Each mirror image enantiomer can be used to prepare in conventional manner antiviral carbocyclic nucleosides of the corresponding enantiomeric configuration, for example as described in Molec Pharm. 37, 395-401 (1990) and J. Med. Chem. 30, 746-749 (1987). This process comprises reducing (xe2x88x92)-(2S, 4R)-4-amino2-cyclopentene-1-carboxylic acid, compound (VIII), the mirror image enantiomer thereof or a mixture of such enantiomers. 
It is preferred that compound (VIII) or its mirror image enantiomer be in the form of a salt, (VIIIa) or (VIIIb). Subsequent references to (VIII), (VIIIa) and (VIIIb) also include the mirror image enantiomers thereof and mixtures of the corresponding enantiomers. Suitable salts (VIIIa) include the lithium, sodium, potassium, magnesium or calcium salts. Most preferred is the sodium salt (Wxe2x95x90Na in structure (VIIIa)). Suitable salts (VIIIb) are those wherein the conjugate acid (XH) of the salt posses a pKa less than two. Suitable salts (VIIIb) thus include the hydrochloride, sulphate, bisulphate, hydrobromide, or organic sulphonic acid salt.
It is further preferred that the salt (VIIIb) be an organic sulphonic acid salt. it is most preferred that the organic sulphonic acid salt is a C1-6 alkyl sulphonic acid salt (e.g. methanesulphonyl) or aryl sulphonic acid salt (e.g. toluenesulphonyl). In structure (VIIIb), X would thus represent most preferably e.g. a methanesulphonate or toluenesulphonate group, respectively.
The present invention also includes the novel compounds of formulas (VIIIa) and (VIIIb) generically and specifically referred to above.
The reducing agent for conversion of (VIII), (VIIIa), or (VIIIb) to (VIIb) or for conversion of the respective mirror image enantiomers is preferably an aluminum hydride, such as diisobutyl aluminum hydride, sodium bis (2-methoxyethoxy)aluminum hydride, lithium aluminum hydride, sodium aluminum hydride, lithium tri-tert-butoxyaluminohydride, etc. Most preferred is lithium aluminum hydride (D. A. Dickman, A. I. Meyers, G. A. Smith and R. E. Cawley, Org. Syn. Coll. Vol VII, 530-533). Advantageously a source of fluoride ion such as NaF (H. Yamamoto and K. Maruoka, J. Org Chem. 1981, 103 4186-4194) isalso used to help release the product from contaminating aluminum following the reduction reaction. Triethanolamine (J. Powell, N. James and S. J Smith, Synthesis, 1986, 338-340) can be used in place of fluoride, but is less preferred.
The solvent for the reduction reaction is preferably an ether such as THF. It is further preferred that water (1-15% w/w) be added to the ether prior to isolation ofthe product, in order to increase the solubility of (VIIb).
In yet a further aspect of the invention there is provided a method of preparing compound (VIIIb), its mirror image enantiomer or a mixture of such enantiomers, comprising reacting (xe2x88x92)-2-azabicyclo[2.2.1]hept-5-en-3-one (IX), its mirror image enantiomer or a mixture of such enantiomers, with one or more equivalents of an acid and one or more equivalents of water. Preferred acids are those with pKa less than two, most preferred are acids that give directlythe salts (VIIIb) described above, e.g. induding methanesulphonic acid and toluenesulphonic acid. 
The reaction temperature can vary between 10xc2x0 C. and 120xc2x0 C., but is most preferably between 30xc2x0 C. and 70xc2x0 C.
The choice of solvent for this hydrolysis reaction can be quite varied, ranging from water to hydrocarbon solvents. Tne preferred solventis the one that will be used in thesubsequent reduction step. In this case, intermediate (VIII or VIIIa or VIIIb) can be used directly, without isolation.
Compound (VIII) and the salts (VIIIa) are prepared from the salt (VIIIb) by contacting it with a base and isolating the product by precipitation, crystallization, evaporation, etc. as is known to those skilled in the art. Almost any base with pKa greater than 3.5 can be used to make (VIII). The salt (VIIIa) must be prepared by contacting (VIIIb) with a base containing (W+). For example, the sodium salt can be prepared by contacting (VIIIb) with about two equivalents of the base sodium hydroxide.
In the present invention, it is also possible to easily remove color and impurities from the salt of intermediate (VIII) by washing it in the reactor (U.S. Pat. No. 4,734,194 Mar. 29, 1988)). Underthe protocol exemplified hereinafter, the toluenesulphonate and methane-sulphonate salts are found to be of particular advantage in that they filter exceptionally quickly.
As a further extension of the present invention, the sulphonic acid salt of comoound (VIII), its mirror image enantiomer or a mixture of such enantiomers, is prepared by performing an oxidative hydrolysis reaction on the Diels-Alder adduct between cyclopentadiene and an alkyl or aryi sulphonyl cyanide (X). 
wherein R7 is C1-6 alkyl or aryl, its mirror image enantiomer or a mixture of such enantiomers. Preferred is where R7 is methyl, phenyl, or tolyl. Most preferred is tolyl.
The literature (J. C. Jagt and A. M. vanLeusen, J. Org. Chem. 1974, 39, 564-566) teaches that the Diels-Alder adduct (X) is a particularly convenient precursor to the lactam (IX) by a hydrolysis reaction. Thus, by the application of an oxidative hydrolysis reaction to Diels-Alder adduct (X), compound (VIIIb) in its further preferred form can be obtained directly, and a step is saved in the overall process to prepare compound (VIIIb).
The oxidative hydrolysis reaction is accomplished by contacting Diels-Ader adduct (X) with at least one equivalent of water, at least one equivalent of an oxidizing agent, and preferably a catalytic amount of an acid.
The choice of solvent can be quite varied. It is preferable to use a solvent that poses a low hazard when combined with the oxidizing agent. Most preferred is to use water as both solvent and hydrolytic agent.
Suitable oxidizing agents are those that do not oxidize a double bond. Preferred are peroxides, most preferred is hydrogen peroxide. One to five equivalents of the oxidizing agent can be used.
In the preferred embodiment where a catalytic amount of acid is used, any acid of pKa less than 3 can be used, but it is preferred that the acid used be the same as the salt of compound (VIIIb) that is formed from the Diels-Alder adduct (VIIIb). For example, if Rxe2x95x90tolyl in the adduct (X), the oxidative hydrolysis gives the toiuenesulphonate salt of compound (VIIIb). In this case, toluenesulphonic acid would be the preferred acid. If Rxe2x95x90methyl in the adduct (X), the preferred acid would be methanesulphonic acid, etc. The amount of acid catalyst can range from 0 to 50 mol c/o relative to the DieisAlder adduct (X).
All of the structures shown above are intended to represent the racemate in addition to the single enantiomer depicted. Thus, the present invention is intended to encompass both the racernates and the pure enantiomers, substantially free of theirmirror-imageisomers.
A compound of formula (I) may be converted into a pharmaceutically acceptable ester by reaction with an appropriate esterifyi ng agent, e.g. an acd halide or anhydri de. The compound of formula (I) including esters thereof, may be converted into pharmaceutically acceptable salts thereof in conventional manner. e.g. by treatment with an appropriate acid. An esteror salt of an ester of formula (I) may be converted into the parent compound, e.g. by hydrolysis.
The following Examples are intended for illustration only and are not intended to limit the scope of the invention in anyway. The term active ingredient as used in the examples means a compound of formrula (I) or a pharmaceutically acceptable derivative thereof.